When a coin characteristic is being tested, the monitoring means has to make a reasonably accurate assessment of the degree of interaction between the coin and a field, to determine whether the coin meets an acceptability criterion. However, one of the fields may be used simply to detect the arrival of a coin and then, so far as that field is concerned, the monitoring means will function simply to detect whether a degree of interaction occurs which is great enough to indicate that an object which might be a coin, which has to be tested, is in the vicinity of that field, in response to which a coin testing sequence of events will be initiated in the apparatus, as is well known. It is also possible for the interaction of a coin with one of the fields to be utilized both for indicating coin arrival and also for testing against an acceptability criterion.
One way of providing an oscillating magnetic field is to place a single inductive coil adjacent to the coin path, that coil being connected as part of a self-excited oscillator circuit such as a Colpitt's oscillator circuit. Another way of producing such a field is to place two inductive coils on opposite sides of the coin path and in register with each other, these being connected together either in series opposing, series aiding, parallel opposing or parallel aiding and also forming part of a self-excited oscillator circuit. Yet another way of providing such a field is to have one coil on one side of the coin path driven by a fixed-frequency oscillator, or by dividing down the frequency of a clock circuit, and to have another coil in register with the first coil and opposed to it across the coin path, the second coil having an oscillating signal induced therein by the transmitted field, which signal will be influenced by the degree of interaction between the coin and the field when a coin passes between the two coils.
The degree of interaction between the coin and the field is detected by monitoring the electrical signal across the coil or coils in a self-excited oscillator arrangement, or by monitoring the signal across the receiving coil in a transmit-receive arrangement. In both arrangements, it may be the amplitude, the frequency, or the phase of the electrical signal that is utilised in determining whether or not a coin is acceptable or, for coin arrival sensing, whether or not a coin is present.
It has been usual, in order for two fields to interact with a coin independently of each other, to space the fields apart, by spacing apart the coils which generate or interact with the fields. This occupies, in general, twice as much space as would a single field arrangement. In coin testing apparatus, compactness is always, and increasingly, desired.
In U.S. Pat. No. 3,918,563, which did not specifically address the question of compactness, it was proposed to have two transmitting coils operating at different frequencies wound on the same core but it was then necessary to add frequency filtering circuitry so as to separate from each other the two frequencies involved to enable them to be individually monitored, for the purpose of applying the desired tests for coin acceptability.